Mackie Powered Speaker Repair

[RubiconJoe]

I am attempting to repair a Mackie (TH-15A) powered speaker's amplifier.  The original fault was that the unit would not power on. 

After opening her up and referring to the schematic I found a few burnt components and replaced them.  The final amplifier chip (TDA7294) as well as the resistor on its output were damaged.  Good news was the amp now powered on (and didn't blow up the new parts).   Bad news the LED that indicates the unit is powered on lights up but then dims almost to off, then back on periodically.  I checked the +- 28V,  +-15V and 3.3V with the all other boards connected, and supply is still fluctuates.  Then disconnecting the amplifier board, supply still fluctuating.   Finally disconnected the pre-amp board and the rails all became solid with correct voltage levels.  Now I'm kind of stuck.  I'm not sure what to suspect on this pre-amp board.  There is nothing obviously damaged and many surface mount components.   Unfortunatly, I do not have much in the way of test equipment, only a DMM.   Any suggestions on how to proceed would be greatly appreciated!


Thank you for anyone who can help!

[CJay]

A picture of the boards is always useful to help debug things remotely

[RubiconJoe]

See attached.  First picture is a picture of the power supply and amplifier board.  Next front and back of pre-amp board, then DSP board that plugs on top of the pre-amp board.

Also pdf of schematic at the link below.

https://bmamps.com/Schematics/Mackie/Mackie_Thump_TH-15A_PCB_Schematics.pdf

Hope these help.


~Joe

[RubiconJoe]

Correction:  There are two connections to the pre-amp board.  One power supply connection with +-15 & 3.3 V, and the 28V connection goes only to the amplifier board.  Then the signal connections between power and pre-amp boards.

After retesting, I have found that voltage fluctuates on all rails only when the power-amp board is connected, either by itself or with the pre-amp connected.  The rails are solid when the pre-amp board is connected alone.  Looks like there is another fault on the power-amp board, not the pre-amp.  Sorry for the confusion.

What would cause my supply to fluctuate?  A bad cap?  If it were a short somewhere, I assume more components would burn.  How can I test for this type of fault?

Thank you in advance!

~Joe

[PedroDaGr8]

Correction:  There are two connections to the pre-amp board.  One power supply connection with +-15 & 3.3 V, and the 28V connection goes only to the amplifier board.  Then the signal connections between power and pre-amp boards.

After retesting, I have found that voltage fluctuates on all rails only when the power-amp board is connected, either by itself or with the pre-amp connected.  The rails are solid when the pre-amp board is connected alone.  Looks like there is another fault on the power-amp board, not the pre-amp.  Sorry for the confusion.

What would cause my supply to fluctuate?  A bad cap?  If it were a short somewhere, I assume more components would burn.  How can I test for this type of fault?

Thank you in advance!

~Joe

Power supply entering protect mode, then coming out of it. Seeing the fault again and re-entering protect mode.

Sent from my LGLS992 using Tapatalk

[RubiconJoe]

Thanks for the reply!  That does makes sense.  Any suggestions on how to further troubleshoot this issue?

[richnormand]

Thanks for the reply!  That does makes sense.  Any suggestions on how to further troubleshoot this issue?

I assume the power amp is the board on the right in your first photo.
Also you mentioned that you have a DMM for checking.
Considering that the power supply chokes up when the power board is in circuit you can assume there a pretty high load from something that should be close to a short.

As a first pass I would measure the resistance between ground and the voltage supply input rails to the power amp board (with PSU disconnected from it).
Look for a shorted/leaky capacitor or diode on the board.
Look at the amp chip itself too. It may have been damaged the moment you plugged the replacement unit in. Again that would show up as a near short.
Using the diode test range on the DMM applies enough voltage to go in conduction so you should repeat the resistance measurements in both directions for diodes transistors and ICs.

[geobee]

Hi there, I have worked on these Mackie units, and I would say the supply is going into protect mode, due to a short in the power amp section. They usually use a digital amplifier IC as the output, and these go down very easily. Not easy to replace unless you have a rework iron with a heat gun unit to extract the old one, even then be careful of board track damage. Best leave this job to a pro workshop, sorry, probably not what you want to hear. I am not a fan of these digital power amp modules, or the switchmode supplies used in musical equipment these days, nut they are lightweight, which is what the consumer wants, and cheaper to produce, less copper etc that a conventional transformer, but more difficult to repair when they turn up their legs so to speak.

[geobee]

Sorry, got the model wrong re it being a digital SM ic as in some of the Mackie models.  But the part about a power amp short would still be my best bet. If the IC has been replaced, it could be a diode shorted or leaky or even, but less likely a cap gone down. Also, check any other transistors on the board, best to remove and check if any funny readings are coming up.

[RubiconJoe]

After a very busy couple of months, I have finally returned to try and repair this speaker's amp.  I am kind of going in circles on this trying to figure out what is wrong here.

Here is what I have tested so far.

Power supply rails (power supply disconnected from all other boards)
+28 V measured +29.3 V
-28 V measured -29.3 V
+15 V measured 14.2 V
-15 V measured -14.2 V
+3.3 V measured +3.2V

Should I be concerned with +-15 V rail being low at +-14V?

Power supply rails (with conn. 3 plugged in, Power amp board only) All voltages remain the same.

Power supply rails (with conn. 11 plugged in, pre-amp board) All voltages start fluctuating as power supply goes into failure mode.

Interestingly enough, when I unscrewed the pre-amp board and pulled it away from the chassis to test, I tried to power it on and the power supply then became stable.  I cannot seem to find any loose connections on this board.  I wiggled everything that wiggles on this board and cannot replicate the problem when the board isn't mounted.  After then screwing the board back onto the chassis, powering it on, the voltages fluctuate again.  Maybe a grounding issue?  I cannot seem to find this fault.

I have also tested all transistors and diodes that I can access with a DMM, across both pre and power amp boards and none are shorted.

Hope someone can point me in the right direction to fix this unit!

Thank you for your help,

Joe

[Armadillo]

With only a DMM, I would segregate the trouble-shooting by eliminating assumption as much as possible. The steps involved qualify the power supply, then the power amp then the pre-amp.

To qualify the power supply;

Remove the power amp and pre-amp from the circuit. Measure the current [in series] available across the +28v and -ve28v line when say a incandescent bulb [ use anything > 100 Watts say 50 + 50 + 50 in parallel ] is connected [briefly] across them. Pass if STABLE current flow. Failed otherwise.  [Proper way is to use programmable DC load bank but of course.].  [no need to bother with the 15v and 3.3v line].

To qualify the power amp include measuring the quiescent current draw at zero input signal [short the input to ground]. [Quiescent Current = ? amp]
Then Connect up the speakers to your amp [without preamp], connect the input to your PC audio out, play audio tone or something [AT LOW VOLUME], Pass if speakers happy, fail otherwise.

To qualify the preamp..... to be continued.

[zepster44]

I have 3 of the Mackie Thump 15a speakers and 2 of them will not put out any volume until you turn the gain up all the way on the back of the speaker.  Once the volume is up, it sounds terrible.  I know it's an issue with the amp because I took my 1 working speaker and connected the amp from that to the cabinet that wasn't working and it sounded just fine.  Any idea what would cause this gain issue on 2 of the speakers?  Is this something I could easily fix with some my shade tree electronics repair skill?  Or can anyone recommend a good repair service?  Cheapest I've found is $119 on Ebay plus $15 for shipping a piece.

thanks
Chris

[firstcolle]

I have 3 of the Mackie Thump 15a speakers and 2 of them will not put out any volume until you turn the gain up all the way on the back of the speaker.  Once the volume is up, it sounds terrible.  I know it's an issue with the amp because I took my 1 working speaker and connected the amp from that to the cabinet that wasn't working and it sounded just fine.  Any idea what would cause this gain issue on 2 of the speakers?  Is this something I could easily fix with some my shade tree electronics repair skill?  Or can anyone recommend a good repair service?  Cheapest I've found is $119 on Ebay plus $15 for shipping a piece.

thanks
Chris

hi,
same issue here... any news?

[SoundTech-LG]

That LEVEL knob looks HUGE and vulnerable to SMACKING during use, and transit, etc. If it does get smacked, it cracks the volume pot behind it. It then becomes a switch, instead of a pot. The switch makes contact at full clockwise rotation. You get full gain, and unless your input signal is drastically reduced, a lot of added distortion. Replace the pot., and then protect from further SMACKING.

[zepster44]

I have 3 of the Mackie Thump 15a speakers and 2 of them will not put out any volume until you turn the gain up all the way on the back of the speaker.  Once the volume is up, it sounds terrible.  I know it's an issue with the amp because I took my 1 working speaker and connected the amp from that to the cabinet that wasn't working and it sounded just fine.  Any idea what would cause this gain issue on 2 of the speakers?  Is this something I could easily fix with some my shade tree electronics repair skill?  Or can anyone recommend a good repair service?  Cheapest I've found is $119 on Ebay plus $15 for shipping a piece.

thanks
Chris

hi,
same issue here... any news?

Still no luck.  One person said it could be the ASP board that needs replacing but, I'm not technically inclined to verify if that is the issue.  instead I contacted a local shop to get one fixed and once I find out their pricing, if its within an acceptable range, I'll get it done and post what they did to fix it on here.
Modify message

[Armadillo]

Same, Check your input signal first; It should be XLR input. Remember turn down the volume control.

https://www.eevblog.com/forum/repair/odd-output-signal-jbl-gto-75-4-amplifier/